Provided herein are water quality instruments containing multiple sensors for measuring a plurality of water-related parameters. The sensors are uniquely configured to have an extremely high form factor so that they may be contained within a housing that minimizes dead space between sensors and within the housing, with the individual sensor ends forming a single continuous sensing surface. This provides a number of functional benefits in the field of multi-parameter sondes and related sensing methods, including for in-situ applications where the total diameter of the sonde instrument is desirably less than 4″ or less than 2″.
The sensors described herein include specially configured high-fidelity and robust turbidity sensors and fluorometers that may be incorporated into a multiparameter sonde, including any of those described in U.S. App. No. 62/077,528 filed Nov. 10, 2014 and U.S. Design application Ser. No. 29/513,888 filed Jan. 6, 2015, which are specifically incorporated herein by reference.
In contrast to the sensors provided herein, much of the existing state of the art in-situ turbidity sensors and fluorometers drift over time and temperature. Most designs use one light source and one detector, such as the systems described in U.S. Pat. No. 8,488,122. The disadvantage of such sensors arises from the light source, such as a light emitting diode (LED), whose brightness changes with temperature. Such change in optical output with change in temperature can show up as a false or erroneous change in the readings of the sensor. Methods exist for compensating for this change by measuring temperature and correcting the change in detector signal with temperature. This is time consuming, is not always accurate, and can be very non-linear, as LED optical output is generally non-linear with temperature.
Other state of the art in-situ turbidity sensors use one LED and two detectors, however, the two detectors are used to measure over two different optical path lengths to achieve a greater dynamic range of measurement. The shorter optical path length is optimized for measuring at high turbidity levels and the longer optical path length is optimized for measuring at low turbidity levels. Devices that measure at two optical path lengths generally require much larger sensor geometries unsuitable for sub-two inch sonde or multi-parameter sondes comprising multiple probes. The designs that work over a shorter path length to optimize range generally suffer from increased noise because of the much smaller excitation/detection volume due to noise statistics related to exciting and detecting in a smaller scattering volume.
The state of the art in-situ sensors generally do not have a built in reference detector and are considered “open loop”. One reason for this is that the in-situ sensors typically have a limited amount of space in them and simply cannot accommodate the added detector in a reliable manner without unduly sacrificing dynamic range and/or signal-to-noise ratio. Usually temperature is measured to attempt to compensate for the LED output changes with temperature. When the LED output changes however, including for non-temperature related reasons, this method does not work. There are “closed loop” systems, but they are typically bulky on-line monitoring type analyzers, not suited for in-situ monitoring applications.
Conventional in-situ turbidity sensors do not have room for the additional optics and electro-optics described herein, especially for sub-2 inch sondes. State of the art sub-2 inch sondes typically have 4 removable sensors each usually between 12 and 16 mm diameter, including as described in U.S. Pat. No. 8,488,122. The round geometry of the sensors greatly limits the optics that can be squeezed into such a sensor.
In view of these limitations, there is a need in the art for fundamentally different optically-based sensor configurations and related optics and electro-optic components that can be tightly packaged for use in multi-parameter and sub-2 inch sondes. Provided herein are turbidity and fluorescent sensors having a fundamental change in structure to address the limitations of conventional sonde sensors, while providing fundamental benefits and attendant improved sonde reliability, durability, and sensitivity.